An internal-combustion engine is provided with a number of cylinders, each of which is connected to an intake manifold via at least one intake valve and to an exhaust manifold via at least one exhaust valve. The intake manifold receives fresh air (i.e., air coming from the external environment) through a supply pipe regulated by a throttle valve and is connected to the cylinders by means of respective intake pipes, each of which is regulated by at least one intake valve.
Recently, there has been proposed the introduction of a tumble system, which is designed to vary the cross section of the intake pipes according to the r.p.m. of the engine itself (i.e., of the angular velocity of rotation of the engine crankshaft). At low r.p.m., the section of passage of the air through the intake pipes is reduced so as to generate turbulence in the flow of intake air and at a point corresponding to the variation in section, which improves mixing between the air and the fuel in the cylinders. In particular, thanks to the presence of this turbulence, which improves mixing, all the injected fuel is burnt, and hence the pollutant emissions generated by combustion are reduced. At high r.p.m., the section of passage of the air through the intake pipes is maximized so as to enable a complete filling of the cylinders and thus enable generation of the maximum power possible.
To vary the section of passage of the air through the intake pipes it has been proposed to provide, within each intake pipe, a throttling body, which is fitted on a common shaft so as to turn between a working position, in which the throttling body reduces the section of the intake pipe, and a resting position, in which the throttling body does not reduce the section of the intake pipe. The common shaft is turned by an actuator, which is designed to control in a simultaneous and synchronous way the position of all the throttling bodies.
In engines currently available on the market, the actuator of the tumble system responsible for controlling the throttling bodies comprises an electric motor having a rotor mechanically connected to the common shaft. However, said arrangement involves a relatively heavy weight, is relatively cumbersome (also on account of the presence of the electronics for driving the electric motor), and presents an acceptable, but not particularly high, level of reliability (in particular, said solution is subject to malfunctioning of the electronics for driving the electric motor). In engines currently available on the market, it has also been proposed to use a pneumatic actuator for driving the common shaft. However, also this arrangement has a relatively heavy weight and above all considerable overall dimensions.